1. Field of the Invention
The present invention relates to a color conversion adjustment method for a color printer which carries out gray correction conversion for correcting the gray balance for image data, and to a gray correction chart used at the time of adjusting or confirming gray correction conversion, and in particular, to a gray correction chart which determines colors of background portions other than color patches and to a color conversion adjustment method which uses a gray correction chart which determines background color of background portions other than color patches.
2. Description of the Related Art
In a color press which uses a rotary press or the like, a color print formed by a so-called dot image is formed. Before this color print is formed, a color print proof image is prepared in advance for a color printer having a simple structure, and correction of the color printing is carried out on the basis of this image. By using this color printer, there is no need to form a printing plate (PS plate), or a lithographic film or the like which are needed only when correction is carried out at the color press. Thus, a plurality of color print proof images can be prepared in a short period of time, and the correction work is made much more efficient.
Before the color print proof image for correction is formed, however, variations in density, which are caused by changes over time, intrinsic differences which occur as a matter of course among color printers of the same type, and the like need to be corrected. Namely, there is the need to carry out density calibration.
For example, in the case of a printer which is designed so as to have a standard gradation output density curve 140 as illustrated by the one-dot chain line in FIG. 15A, the output density curve 140 varies to, for example, an output density curve 142, illustrated by the solid line, due to intrinsic differences among individual printers or changes over time or the like. In this case, even if printer signal P.sub.1 or P.sub.2 is input to the data output section of the printer in order to obtain an output density of D.sub.1 or D.sub.2, the density which is actually outputted is D.sub.1 ' or D.sub.2 ', and in this state, a correct color print proof image cannot be outputted.
For example, a conversion curve 150, illustrated in FIG. 15B, is made use of to obtain the output density of the standard gradation. The printer signals P.sub.1, P.sub.2 are converted to signals P.sub.1 ', P.sub.2 ' by the conversion curve 150. As illustrated in FIG. 15A, the correct output densities D.sub.1, D.sub.2 are obtained by the signals P.sub.1 ', P.sub.2 ' being inputted to the printer data output section having the characteristic expressed by the output density curve 142.
Conventionally, such correction of color density was carried out by implementing, for example, the following steps.
Step 1: Outputting of Density Calibration Reference Chart
In step 1, a density calibration reference chart is printed (outputted) onto a first recording sheet. The density calibration reference chart is obtained by converting the dot percent data (dot % data) for each of C (cyan), M (magenta), Y (yellow), and K (black) in the chart data, by using a 4-D (four-dimensional) conversion table for color correction which is built-into the color printer. (Hereinafter, this table will be referred to as the "reference color conversion 4-D table".) In the density calibration reference chart, color patches for each of C, M, Y, K are printed (outputted) by changing the dot % density for each of C, M, Y, K in equal stages.
By comparing (color patches of) this density calibration reference chart with (color patches of) a reference calibration chart prepared in advance, the density difference between the actual printer output value and a theoretical value is obtained. A 1-D (one-dimensional) conversion table for density calibration for correcting this density difference is selected. (Hereinafter, this table will be referred to as the "density calibration 1-D table".)
Step 2: Outputting of Density Calibration Confirmation Chart.
In step 2, a density calibration confirmation chart is printed (outputted) onto a second recording sheet. The density calibration confirmation chart is obtained by converting chart data using the reference color conversion 4-D table and the density calibration 1-D table which was selected in step 1.
By using the outputted density calibration confirmation chart, it is confirmed that the density corrected by the density calibration 1-D table is equal to the reference density. Step 3: Outputting of Gray Correction Reference Chart
In step 3, in order to confirm the fine gray balance at the time of combining the respective colors of C, M and Y whose densities have been corrected, a gray correction reference chart is printed (outputted) onto a third recording sheet. This gray correction reference chart is obtained by converting chart data for gray correction by using the reference color conversion 4-D table and the density calibration 1-D table selected in step 1. In this gray correction reference chart, for example, a plurality of color patches in which C is at a constant density and the densities of M and Y are varied bit-by-bit are disposed around a color patch in which C, M, and Y are combined at equal densities.
The color patch having the best gray balance is selected from the outputted gray correction reference chart. This color patch is compared with the color patch in which C, M and Y are combined at equal densities in order to determine the extent of the density difference in M and Y colors between these two color patches. The gray correction 1-D table is selected on the basis of this density difference.
Step 4: Outputting of Gray Correction Confirmation Chart.
In step 4, a gray correction confirmation chart is printed (outputted) onto a fourth recording sheet. The gray correction confirmation chart is obtained by converting the chart data for gray correction by using the following three tables: the reference color conversion 4-D table, the density calibration 1-D table selected in step 1, and gray correction 1-D table selected in step 3.
By using the outputted gray correction confirmation chart, it is confirmed that the gray balance, which has been corrected by the gray correction 1-D table, is optimal.
In this way, the color density and the gray balance are adjusted at the color printer. After adjustment, a color print proof image is printed on the basis of a combination table obtained by combining the reference color conversion 4-D table, the density calibration 1-D table selected in step 1, and the gray correction 1-D table selected in step 3. On the basis of this image, color correction of the color press is carried out.
However, in the above-described conventional method of adjusting a color conversion table, the judgment criteria for visually judging the fine color balance of the gray correction chart are not clearly prescribed. Therefore, there is a drawback in that objectivity in judging the gray balance cannot be ensured.
In order to overcome this drawback, there is a method in which a black color of a predetermined density is printed on a different paper than the gray correction chart, this black color is used as a reference for judging the gray balance. However, with this method, another recording sheet for the black color must be prepared, and therefore, the work efficiency deteriorates. Further, gray balance may deteriorate due to a difference in the paper used, however, when the deterioration of the gray correction chart paper is different to the deterioration of the black color paper, then the gray balance judgment is incorrect.